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Credit for awesome summary Attack on Titan Season 4 Wikipedia.
Three years ago, Yelena along with other Marleyan deserters offer to help the Eldians on behalf of Zeke Jaeger to carry out the Rumbling in order to bring peace to Eldia. For the Rumbling, a Titan of royal blood and the Founding Titan are required, and Zeke wants Eren and his friends to cooperate with him to execute his plan.
Yelena and Onyankopon, another former Marleyan soldier, provide them with schematics of guns and advanced weaponry. Using Yelena to lure in more scout ships, the Survey Corps recruits Marleyan soldiers as labor to build an expansive set of docks for the island.
Eren, Armin and Mikasa discuss the next steps they should take. Armin believes in garnering peace with Marley due to soldiers like Yelena, but Eren believes Marley should be destroyed by force.
In the present day, a Marleyan chef named Nicolo, who was friends with a now deceased Sasha, grieves for her death and makes peace with Sasha's mourning family. Later, Mikasa sits by Sasha's grave as she reflects that losing would spell death for them.
Armin remarks to the still-crystallized Annie on how things could have been different if they had acted differently before, and begs for her to wake up. An incarcerated Eren reflects that fighting will be the only way to achieve victory.
NOTE: This episode is from Volume 26 of the manga.